It sounds like you may be dealing with a little bit of depression. It's very normal within the first year of having a baby. I think, especially during that first year, that babies have so many needs we meet as moms that it seems like we don't have time (or energy) to take care of ourselves. Little things can make a big difference, though. Getting enough sleep (as in, going to bed at a reasonable time), even if you want to stay up a little later after the baby goes to bed, is one of thing that can make a world of difference. Eating breakfast every day and not skipping other meals will also improve your day considerably. I'm not a breakfast person myself, but my day is shot if I don't eat at least a small meal (high in protein, low in carbs) in the morning. Even if you don't have time to sit and eat a full meal, having small meals and healthy snacks throughout the day can keep you from having ups and downs in your energy level due to the changes in your blood sugar. Keeping a water bottle with you throughout the day can also be a big help. And, yes, find at least 15 minutes a day to exercise. It's not a huge commitment but makes you feel SO much better.
Even when you don't have anywhere to go and may not feel like you have time, take the time to shower, dress in something more than sweats and t-shirts, and at least pull your hair up. If you like to wear makeup, put on at least a bare minimum. And keep your shoes on even when you're walking around the house. It may sound silly, but when you are more put together, you'll feel more energetic and less like, well, the tired mom in sweats and crazy hair. Speaking from experience!
Otherwise, all you can do is schedule an appointment with your doctor and make sure there aren't underlying causes like anemia or thyroid issues. Best of luck!